In the Second World War, following the evacuation of British and French soldiers from Dunkirk and the French surrender on 22 June, 1940, Hitler believed the war was practically over and the British, defeated on the continent and without European allies, would quickly come to terms. Although there was an element of British public and political sentiment favouring negotiated peace with a clearly ascendant Germany – among them the Foreign Secretary, Lord Halifax – the recently-installed Churchill nonetheless refused to consider an armistice with Hitler's Germany.Churchill's skillful use of rhetoric hardened public opinion against a peaceful resolution and prepared the British for a long war.

On 16 July, in an effort to finish the war in the west, Hitler ordered the rapid preparation of a plan to invade Britain, "Directive No. 16; On the Preparation of a Landing Operation against England". Directive No. 16 read, in part, as follows:

Since England, despite its militarily hopeless situation, still has not shown any signs of being prepared to negotiate, I have decided to prepare a landing operation against England and, if necessary, carry it out. The objective of this operation is to eliminate the English home country as a base for the continuation of the war against Germany...

2) Included in these preparations is the bringing about of those preconditions which make a landing in England possible;

a) The English air force must have been beaten down to such an extent morally and in actual fact that it can no longer muster any power of attack worth mentioning against the German crossing. (italics added)

Hitler hoped to frighten Britain into peace using the preparations as a means to apply pressure. Prior to this, on 11 July, Admiral Raeder had told Hitler invasion could only be contemplated as a last resort, and only then with full air superiority. The Kriegsmarine (German Navy) had been nearly crippled by the Norwegian Campaign, with many of its ships having been sunk or damaged, while the Royal Navy had over 50 destroyers, 21 cruisers and eight battleships in the British Home Fleet.However, it is often overlooked that the Royal Navy had deployed 39 destroyers during the Dunkirk evacuation, losing six, with a further 19 damaged and put out of service; even so, by early July, 40 destroyers were disposed between the Humber and Portsmouth. There was little the weakened Kriegsmarine could do to stop the Royal Navy from intervening. The only alternative was to use the Luftwaffe's dive bombers and torpedo bombers, which required air superiority in order to operate effectively.

Although Hitler agreed with Raeder, he nevertheless ordered all services to make preparations for an amphibious assault once air superiority had been achieved.The plan was prepared by the Oberkommando der Wehrmacht (OKW) or High Command of the Armed Forces. The operation, code-named Seelöwe ("Sealion"), was scheduled for mid-September 1940 and called for landings on the south coast of Great Britain, backed by an airborne assault. All preparations were to be made by mid August.

Neither Hitler nor OKW believed it possible to carry out a successful amphibious assault on Britain until the RAF had been neutralised. Some historians, such as Derek Robinson, have argued that an invasion could not have succeeded even then, contending that the massive superiority of the Royal Navy over the Kriegsmarine would have made Sealion a disaster and the Luftwaffe would have been unable to prevent decisive intervention by British cruisers and destroyers, even with air superiority.This perspective was shared by Grossadmiral Karl Dönitz, who believed air superiority was "not enough". Dönitz believed, "we possessed neither control of the air or the sea; nor were we in any position to gain it". Grossadmiral Erich Raeder, Commander-in-Chief of the Kriegsmarine at the time, was more optimistic, believing that air superiority might make a successful landing possible. He maintained it would be a very risky operation and required "absolute mastery over the Channel by our air forces".